This invention relates to a particle filter and more particularly to a backwash outlet assembly associated with the filter adapted to restrict loss of filter media during a backwash cleaning operation. The invention also includes a method whereby restriction of the loss of filter media referred to can be achieved.
Particulate filters operate by passing a liquid frequently water, to be filtered through particles or filter media wherein filterable impurities adhere to or are caught between the particles and are thereby removed from the liquid. At periodic intervals it is necessary to clean the filter media to precent clogging and improve efficiency. This is normally achieved by what is usually a two stage process known as backwashing. Firstly the filter media is agitated to thereby loosen the impurities from the media. The agitation is conveniently performed by bubbling air through the filter media. Secondly backwash liquid is passed through the filter media under reverse flow thus expanding the filter bed and carrying the dislodged particles to a suitable waste outlet for the filter. Conventionally the backwash outlet assembly includes at least one weir extending above the filter bed. This is done in an attempt to prevent filter media loss during backwashing of the filter.
The less the depth between the sill of the weir and the tip of the filter media, generally speaking, the higher the efficiency of the washing system since there is less dilution in the body of liquid above the filter media during the backwashing process and also the impurities have to travel a lesser distance to waste. However, if the said depth of the weir above the top of the filter media is reduced too greatly, then the media being expanded tends to be carried to waste with the impurities over the weir sill during the backwashing process. This is particularly true of mixed media filter arrangements in which the top layer of filter medium may be material such as anthracite having a lower specific gravity than sand.
A further very important factor that has a bearing on the carrying of filter media to waste applies particularly when air agitation is used prior to backwashing or when for any reason air pockets are present in the media. Under such circumstances of backwashing, residual air that has remained in the filter floor distribution system or the filtering media discharges with the backwash liquid stream and tends to lift media particles above the general expanded level of the media, and if this occurs in a critical zone preceding the weir sill, appreciable quantities of filter media will be carried to waste with the backwash liquid.
It is an object of this invention to provide a particulate filter backwash outlet in which the problem relating to loss of filter media is, at least, diminished.